Casting core with crossover bridge

ABSTRACT

A casting core for use in investment casting having at least one gap between two separate core material parts which define separate adjacent cavities in the finished product, wherein the two core material parts are connected by at least one crossover bridge fixing the position of the two core material parts relative to each other, wherein the crossover bridge has a cross section whose wall contour has at least one convex and at least one concave section.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Application No. EP17176183 filed 15 Jun. 2017, incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to a casting core for use in investment casting comprising at least one gap between two separate core material parts which define separate adjacent cavities in the finished product, wherein the two core material parts are connected by at least one crossover bridge fixing the position of the two core material parts relative to each other.

BACKGROUND OF INVENTION

Investment casting is a metal-forming technique in which liquid metallic material is poured into a cavity of refractory material that is an exact inverse duplicate of the desired product. To shape any details inside this product, a casting core has to be incorporated which at the end of the production process is removed by chemical or physical means. Said casting core is not identical with the refractory material forming the outer walls of the later finished product. The casting core mainly allows to shape internal surfaces which are typically not visible from outside. One such example is e.g. a gas turbine airfoil whose inner cooling channels are formed with the help of an investment casting core.

Complex inner structures of such investment casting products are formed with separate core material parts between which a gap is arranged which is filled with liquid metal during the casting process. As these separate core material parts are typically not connected directly, they require means of fixation to provide a stable overall core structure. Such fixation means, however, should not introduce unwanted features during the casting process. Thus, the means of fixation are normally kept small, but big enough to provide stability to the overall core structure.

One typical means of fixation is a crossover bridge which is connected on one end to one separate core material part and the other end to the other separate core material part.

The function of these crossover bridges is only to provide stability to the overall core structure before and during the casting process. They differ from other channels, which e.g. have a specific technical function, e.g. cooling channels. Crossover bridges are typically kept as small as possible in order to reduce negative or unwanted impact on the product. In the finished investment casting product, they typically do not have any desired function.

Because crossover bridges introduce additional and unwanted holes into the inner structure of the finished product, they also introduce undesired stress points which can be a reason for material failure in the finished product. E.g. in gas turbine airfoils due to their high operation temperatures and immense physical forces which are experienced such stress points have an undesired impact on the overall lifetime of the airfoil and thus on the servicing intervals which have to be observed.

Stress point analysis of the Applicant has shown that in particular corner shaped hole sections in the finished products are particularly prone to develop stress points and lead to material failure. For this reason, most of the crossover bridges used for investment casting have a circular or elliptical shape in cross section. Such shapes, however, are still not sufficiently safe as even such shapes develop undesired stress points under high temperature operation conditions.

It is therefore a technical necessity to provide for improved investment casting cores which have at least two separate core material parts which are connected by at least one crossover bridge fixing the position of the two core material parts relative to each other before and during the casting process.

SUMMARY OF INVENTION

To address these problems, a casting core for use in investment casting is provided, which comprises at least one gap between two separate core material parts which define separate adjacent cavities in the finished product, wherein the two core material parts are connected by at least one crossover bridge fixing the position of the two core material parts relative to each other, wherein the crossover bridge has a cross section whose wall contour has at least one convex and at least one concave section.

It has to be pointed out that a casting core for use in investment casting is an investment casting core, i.e. a casting core which can be used under high temperature conditions and withstand the mechanical and thermal stresses during the casting process. In particular, such casting cores are not a lost-wax casting core or any other casting core which would get damaged in the temperature environment during the investment casting process.

Further, the two separate and adjacent cavities which are defined by the two separate core material parts can be closed or open cavities. The term cavity, thus, has to be interpreted broadly.

It has to be further pointed out that the two separate adjacent cavities in the finished product are typically functional cavities, i.e. they both have a technical function. In a gas turbine airfoil e.g. these cavities could be cooling cavities.

These two separate cavities are formed by respective separate core material parts which would not be sufficiently fixed relative to each other and could not withstand the mechanical forces during the casting process if the crossover bridge or the crossover bridges was/were not existing.

Due to the shape of the cross section of the crossover bridge whose wall contour has at least one convex and at least one concave section, thermal and mechanical stresses are distributed more evenly over the entire wall contour, thus resulting in less stress points or in stress points whose stress build-up during operation conditions is less than in stress points known from prior art. Also, the opposite convex and concave curvatures allow directing the stress away from the region at which it is introduced into the wall contour.

This measure not only enhances material integrity and the lifetime of the finished investment casting product but also reduces the costs for service and replacement after material failure.

A particular aspect of the present invention is that the cross section of the crossover bridge is the cross section perpendicular to the longitudinal extension of the crossover bridge. The longitudinal extension follows the extension of the crossover bridge from one separate core material part to the next one. Typically, this cross section remains unchanged along the longitudinal extension. If, contrary to present invention, the shape of the crossover bridge would be cylindrical, the longitudinal extension would typically be the extension along the cylindrical axis of symmetry. In other words, the longitudinal extension follows a line which is representative of the crossover bridge if it was shrunk down to a simple line connecting the two separate core material parts.

According to another embodiment of the casting core the cross section of the crossover bridge does not have an extension which is larger than 20.0 mm. This extension is the largest possible extension from one point of the wall contour to another point of this wall contour in the same cross section. In most applications, the extension is smaller than 12.00 mm.

In another aspect of present invention, the wall contour of the cross section has exactly two convex sections and two concave sections. The length of these two convex sections along the wall contour may be equal. Also, the length of the two concave sections along the wall contour may be equal. Typically, the resulting cross section shape has a mirror symmetry. This cross section is typically called “peanut” shaped. Further, the smallest distance from one point of the wall contour to another point of this wall contour is 3.0 mm.

Advantageously, all of the wall contour of the cross section is curved. This results in improved stress distribution properties in the finished products in the region of the crossover bridge and the reduction of stress build-up. A curved region differs in particular from any straight region, which does not have any curvature.

Further, the material of the crossover bridge may be different from the material of the two separate core material parts. In particular, the material may be quartz. The crossover bridge can also be introduced into the two separate material parts of the casting core after the casting cores have been finally been produced, however, it is normally more practical to introduce the crossover bridge during the production process of the casting core.

According to another aspect of present invention, the crossover bridge and at least one of the two separate core material parts are made in one piece. This allows the production of the casting core to be automated and cast or printed in one single piece.

In one particular embodiment, the at least one of the two separate core material parts comprises ceramics or are made of ceramics.

In a specific embodiment, the casting core is a casting core for a gas turbine airfoil. The two separate adjacent cavities are thus typically air cooling cavities which are positioned inside the airfoil.

Finally, the above mentioned problems are also addressed by an investment casting product which is produced by using a casting core according to one of the previous or following embodiments. In particular the investment casting product is a gas turbine air foil. The product has at least two adjacent cavities with a connecting channel between them resulting from a crossover bridge as described above. The connecting channel has a cross section whose wall contour has at least one convex and at least one concave section.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned attributes and other features and advantages of this invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein

FIG. 1 shows a 3D side view of a casting core 1 for use in investment casting with two separate core material parts 12 and 13 connected by a crossover bridge 15.

FIG. 2 shows a cross sectional view through the crossover bridge 15 of FIG. 1 according to prior art in a cross sectional plane coinciding with the double arrow A.

FIG. 3 shows a cross sectional view through the crossover bridge 15 of FIG. 1 according to an embodiment of present invention in a cross sectional plane coinciding with the double arrow A.

FIG. 4 shows a side view of an inventive investment casting product 40 which could be produced in an investment casting process using a casting core 1 with separate core material parts 12 and 13 as e.g. shown in FIG. 1.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a 3D side view of a casting core 1 for use in investment casting with two separate core material parts 12 and 13 connected by a crossover bridge 15. The casting core 1 comprises at least one gap 11 between the two separate core material parts 12 and 13. The core material parts 12 and 13 will define separate adjacent cavities in the casting product. The two core material parts 12, 13 are connected by at least one crossover bridge 15 fixing the position of the two core material parts 12 and 13 relative to each other.

The crossover bridge 15 connects on one end to one core material part 12 and on the other end to the second core material part 13. The crossover bridge may be sunk into the material of the core material parts 12, 13 or it may be attached and connected to them in any other suitable form.

The cross section 20 of the crossover bridge 15 is shown in more detail in FIG. 2. The cross section 20 is taken in a plane which cuts the crossover bridge 15 in gap 11 between the two core material parts 12, 13 and which coincides with the double arrow A shown in the figure. The plane cuts through the crossover bridge 15 perpendicularly to its longitudinal extension, thus, defining a symmetrical cross section 20. The cross section 20 is defined by the wall contour 21 which is of the shape of a rectangle with rounded edges. The walls of the core material part 12 lying behind the cross sectional plane are indicated by a dotted line. Proportions might be different when compared to the proportions in FIG. 1.

In the final investment casting product 40 inverse wall contours exist, with the crossover bridge 15 replaced by a connecting channel 41. The rounded edges in this connecting channel 41, however, can result in stress points 25 in the finished casting product 40. In FIG. 2, the location of these stress points 25 is indicated where they will occur in the future finished investment casting product 40. When used in a regular operating environment, these stress points 25, however, can lead to material failure and the casting product 40 can show fissures or cracks which can reduce significantly its life time.

FIG. 3 in contrast shows a cross sectional view of the crossover bridge 15 according to an embodiment of present invention in a plane coinciding with the double arrow A of FIG. 1. The crossover bridge 15 has a cross section 20 with a wall contour 21 that has exactly two convex sections 31 and two concave sections 32. The resulting cross section 20 has a “peanut” shape. The cross section 20 is symmetrical with two planes of symmetry. The lengths of the two convex sections 31 along the wall contour 21 are equal. Also, the lengths of the two concave sections 32 along the wall contour 21 are also equal. Further, the smallest distance from one point of the wall contour to another point of this wall contour is not more than 3.0 mm. This smallest distance is defined by the smallest distance between the two convex sections 31.

Due to the wall contour 21 of the crossover bridge 15 the resulting connecting channel 45 in the finished product 40 has a similar cross section and in the case of use of the product 40 in its regular operating environment thermal and mechanical stresses are distributed more evenly over the entire wall contour. This reduces the formation of stress points 25 or diminishes the stress build-up in smaller regions at the wall contour lines. The curvature along the wall contour allows the transfer of stress to neighbouring regions, thus reducing the increase of stress in one single small region.

FIG. 4 shows a side view of an investment casting product 40 which may be produced in an investment casting process using a casting core 1 with separate core material parts 12 and 13 as e.g. shown in FIG. 1. The shown product is a gas turbine airfoil which has inner wall divisions 41. The wall divisions 41 result from metallic material during the casting process filling the region of gap 11 in the casting core 1. In consequence, the crossover bridge 15 fixing the two core material parts 12 and 13 relative to each other leaves an undesired connecting channel 45.

The inner wall divisions 41 define cooling channels in which cooling air is transported, allowing the airfoil to be cooled from inside. This cooling mechanism makes it possible to operate the airfoil under harsh operating conditions at temperatures well above 1000° C.

However, some of the cooling capacity of the inner cooling system is lost because of the connecting channels 45 in the wall divisions 41, allowing cooling air from one side of the wall division 41 to flow over to the other side reducing the overall cooling effectiveness. Further, due to the high temperatures and the high mechanical stresses during operation stress can build up at certain points 25, possibly leading to material failure. To avoid this, the cross section 20 of the crossover bridge 15 of the casting core 1 is adapted to allow the stress to be distributed along the contour walls of the connecting channels 41, diminishing any dangerous stress build-up. 

1. A casting core for use in investment casting comprising: at least one gap between two separate core material parts which define separate adjacent cavities in the finished product, wherein the two core material parts are connected by at least one crossover bridge fixing the position of the two core material parts relative to each other, wherein the crossover bridge has a cross section whose wall contour has at least one convex and at least one concave section.
 2. The casting core of claim 1, wherein the cross section is the cross section perpendicular to the longitudinal extension of the crossover bridge.
 3. The casting core of claim 1, wherein the cross section of the crossover bridge does not have an extension which is larger than 20.0 mm.
 4. The casting core of claim 1, wherein the wall contour of the cross section has exactly two convex sections and two concave sections.
 5. The casting core of claim 1, wherein all of the wall contour of the cross section is curved.
 6. The casting core of claim 1, wherein the material of the crossover bridge is different from the material of the two separate core material parts.
 7. The casting core of claim 1, wherein the crossover bridge and at least one of the two separate core material parts are made in one piece.
 8. The casting core of claim 1, wherein at least one of the two separate core material parts comprises ceramics or are made of ceramics.
 9. The casting core of claim 1, wherein the casting core is a casting core for a gas turbine airfoil.
 10. An investment casting product, wherein the product is produced by using a casting core according to claim
 1. 